Stoichiometric LiFePO4 cathode material has been discussed for replacing LiCoO2 type cathode material for lithium ion application because of the potentially lower cost (Fe replacing Co) and the safer operating characteristics of the material (no decomposition of the material during charging). However, present processing issues make the stoichiometric LiFePO4 material expensive and difficult to produce. Presently, LiFePO4 materials suitable for lithium ion battery applications require the synthesis utilizing high temperature heat treatment (>600° C.) under an inert atmosphere. Also, in order to increase the conductivity of the material, electrically conductive carbon is usually used for enhancing the conductivity and therefore the electrochemical properties of the synthesized material. The use of the inert atmosphere is a key factor that assures the good quality of the materials because of its importance in relation to residual carbon in the material. None of the prior art teaches how to synthesize LiFePO4 material in air, without a protective atmosphere, and how to provide good conductivity of the material and thus good electrochemical properties of a cathode formed of the material.
It is known to use olivine structured material to be the active material for a battery cathode, such as in U.S. Pat. No. 5,910,382. Also U.S. Pat. Nos. 6,723,470, 6,730,281, 6,815,122, 6,884,544, and 6,913,855, in general, teach methods and precursors utilized for the formation of stoichiometric LiFePO4, or the substitution of cations for Fe. The above publications only show how stoichiometric olivine structured materials having different cation substitutions are synthesized. None of the prior art teaches how to synthesize phosphate materials having a defective crystalline structure, in air, which have consistent good electrochemical properties for use as an active material in a cathode of a Li-ion battery.
In general, defects in the crystalline structure of a material can affect the electrochemical property of the synthesized material drastically. A classical example is the synthesis of stoichiometric LiNiO2. A deficiency of lithium would lead to Ni mispositioned on the Li sites therefore retarding the diffusivity of Li drastically and causing the loss in capacity at certain rates. The influence of electrochemical properties caused by misposition of Ni on Li sites has been studied by Chang et al. (Solid State Ionics, 112 (1998) 329-344), which is hereby incorporated herein by reference. Additionally, the concentration of the defects can be affected by different processing precursors and processing protocols. For example, a solution processed precursor would in general possess higher reaction kinetics compared to conventional solid state processes and therefore exhibit lower defect concentration. The reason could be attributed to the fact that LiNiO2 undergoes a decomposition reaction that causes loss of Li during heat treatment. As a result, proper precursors that render high formation kinetics would thus decrease the defect concentration of the synthesized material (Chang et al., Journal of the Electrochemical Society, 149 (2002) A331-A338; 149 (2002) A1114-A1120), which is hereby incorporated herein by reference. In the present example, although defects can physically retard the diffusivity of Li, the electronic structure of the material could also be affected by the presence of defects and thus the electrical conductivity of the resultant material. It is thus shown that factors such as precursors, processing environment, processing protocols and the kinetics of the reaction to the materials would affect defect concentration and the properties of the resulting material. In the present invention, a family of defective lithium transition metal phosphate material that can be synthesized at low temperature in air atmosphere possessing excellent rate and cycling capability is created. The formation of defects is caused by incorporating various lithiated transition metal oxides with distinct stoichiometry.